Small diameter glass capillary tubes are commonly used in medicine to draw blood or other fluid samples from a small puncture wound made in a patient's finger or other anatomically necessary site. These glass tubes typically have an inside diameter on the order of 0.021" and an outside diameter on the order of 0.060". Typically, the wall thickness of such a tube is about 0.0195" with a cut length of 3.000". Although tubes of different sizes can be used depending on blood type and type of test performed, all the tubes are very small-diameter tubes which draw blood by virtue of the inherent capillary action of the tube.
Glass tubes operative by virtue of the inherent capillary action of the small bore are called "hematocrits" and are used in conjunction with charts which show visually post-test results by holding the column of blood against the chart thus providing a simple, quick and highly accepted test procedure to determine plasma level as is well known.
Although hematocrits have been used for many years not only to take blood samples but also to perform post-test and sample blood analysis, there are a number of disadvantages to the use of glass tubes. One disadvantage is that the tubes are not disposable readily and cannot be incinerated in conventional hospital or other common institutional incinerators. Thus, once used and possibly contaminated with blood which may carry hepatitis or other viruses such as those identified with AIDS, these tubes are taken to local landfills where they can pose a continuing health hazard.
Another disadvantage to the use of glass hematocrits is that they are fragile and may be broken, cracked, or chipped easily. Consequently, it is possible for the user to come into intimate contact with virus-contaminated blood and possibly become infected. Even though latex gloves may be used when taking blood samples, an obvious hazard is that a chipped tube could puncture the glove inadvertently and break through the skin of the user.
Hematocrits are also difficult to produce to close tolerances. Glass is inherently fragile, silica-based and subject to wide fluctuations in temperature during processing. These factors create variations in the inner and outer dimensions of such small diameter tubes. Such variations compound errors in blood sample tests which must be measured precisely.